Project Summary/Abstract Allograft rejection is characterized by effector CD4+ T cell activation in response to donor antigen and an intense cellular and humoral attack on the graft. However, multiple intracellular signals within CD4+ T cells operate co-incidentally to control and regulate effector alloimmunity, and it is proposed that this process of immunoregulation is a vital and perhaps more potent component of the response. The neuropilin (NRP) receptors, NRP-1 and NRP-2, are single spanning transmembrane glycoproteins that were initially discovered as chemorepulsive mediators of axonal guidance. However, they are now recognized to function in a wide range of important biological processes including migration, angiogenesis, and cell growth. In addition, they have recently been shown to have pluripotent functions in the immune response as well as in tumor initiation and progression. Over the past decade, several groups have identified critical functions for NRP-1 in immunity, notably related to its expression on induced Tregulatory cells where it functions to augment immunoregulation. In contrast, much less is known about the immunological function of NRP-2 and it has only recently been demonstrated to be expressed on immune cells. In preliminary studies, we find that NRP-2 is expressed on subsets of both human and murine CD4+ effector and regulatory cells. Furthermore, we find that Semaphorin3F, a well established ligand for NRP-2, is potent to inhibit PI-3K activity as well as mTOR signaling that are reported to modulate CD4+ T cell activation. Also, we find that CD4+ T cells from NRP-2 knockout mice mount enhanced effector responses following activation in vitro and in vivo. These observations have shaped a working model whereby the induced expression of NRP-2 on CD4+ T cells functions to modulate activation, and they suggest that its biological effects have consequences for the outcome of the alloimmune response. Our objectives in this R01 are to: 1), identify how NRP-2 modulates regulatory signaling responses and cell metabolism in CD4+ subsets, 2), evaluate whether Semaphorin3F serves to regulate CD4+ T cell activation responses, and 3), evaluate whether Sema3F-NRP-2 interactions can be exploited in vivo to augment immunoregulation post transplantation and/or to induce Teffector responses that limit cancer growth. We will test the hypothesis that CD4+ T cell NRP-2 interacts with Semaphorin3F and/or additional ligands to modulate Teffector cell activation and to augment Tregulatory cell function and immunoregulation following transplantation. We propose two specific aims in which we will: 1), identify the mechanism of NRP-2-induced regulatory signaling and the consequences of NRP-2 loss in CD4+ T cell subsets, and 2), determine the function of CD4+ T cell NRP-2 in the prevention of rejection and in long-term allograft survival. Collectively, these innovative studies will have broad scientific and biological implications of great significance and relevance in immunity.